That's bad because Yahoo has only four big products: homepage, finance, email, and sports – and sports traffic is driven mainly by fantasy sports.

In the fall, Sundays are the biggest days for fantasy sports.

Of those four, only finance and sports translate well to mobile, where Internet usage is going.

So it's important that Yahoo protect the franchise. It's already been weakened thanks to big competition from ESPN.com and the leagues themselves.

One fantasy football user who got a note from Yahoo about the downtime sent us an email that said: "I've played Y! Fantasy Football for 10yrs and this is the first email like this I have ever received. "

Anyway, it was the first big mess-up during Marissa Mayer's tenure at Yahoo, and last night she tweeted: "Yahoo! Fantasy Fans, here's what happened Sunday," and linked to this FAQ explanation:

Q. Why were players unable to access Yahoo! Fantasy sports on November 11th?A. We have giant machines called "filers" that store all of the data about your teams and leagues. These filers serve millions of files every hour for our games, and normally our machines can handle this with no problem. Recently, we discovered a hardware issue in one of the filers that caused the other one to overload. We replaced some hardware, re-configured the setup, and did some testing. However this Sunday – at approximately 12:15 pm ET – the new configuration failed. This created an overload on storage capacity and took the Fantasy part of our site down.

We had dozens of engineers from Yahoo! and our storage vendor working together to determine the cause and fix it. Over the course of the morning and early afternoon, we repeatedly thought we were close to a solution, but the problem persisted. After multiple attempts, we decided to move our mobile apps to a back-up data center to get the web version of the game back up and get the mobile apps up in a "read-only" state – meaning you could see scores and data, but not edit lineups or post to message boards. By Monday afternoon, after a thorough system stress test, we moved mobile apps back to a fully functional state in the original datacenter in time for Monday Night Football.